Question

Some context for this code,
what I am trying to do is create a validation warning if the input containts “” as a value,
so if an error is displayed it displays the message.
If it is valid it does not display a message,
so how do I remove the message when the textField is valid?

The message is a JPanel that contains a JLabel with the text in it,
I add this this JPanel to the frame when it is not valid,
and I am trying to remove it when it is valid.

So what am I doing wrong here?
I am at a basic level with Swing.

import java.awt.EventQueue;

import javax.swing.JFrame;
import javax.swing.JPanel;
import javax.swing.JLabel;
import javax.swing.JTextField;
import java.awt.Color;
import java.awt.Font;
import javax.swing.JButton;
import java.awt.event.ItemListener;
import java.awt.event.ItemEvent;
import java.awt.event.MouseAdapter;
import java.awt.event.MouseEvent;

public class Test {

        private JFrame frame;
        private JTextField textField;
        private JTextField textField_1;

        /**
         * Launch the application.
         */
        public static void main(String[] args) {
                EventQueue.invokeLater(new Runnable() {
                        public void run() {
                                try {
                                        Test window = new Test();
                                        window.frame.setVisible(true);
                                } catch (Exception e) {
                                        e.printStackTrace();
                                }
                        }
                });
        }

        /**
         * Create the application.
         */
        public Test() {
                initialize();
        }

        /**
         * Initialize the contents of the frame.
         */
        private void initialize() {
                frame = new JFrame();
                frame.setBounds(100, 100, 401, 232);
                frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
                frame.getContentPane().setLayout(null);

                JPanel panel = new JPanel();
                panel.setBounds(10, 11, 330, 94);
                frame.getContentPane().add(panel);
                panel.setLayout(null);

                JLabel lblNewLabel = new JLabel("Firstname :");
                lblNewLabel.setBounds(10, 11, 104, 14);
                panel.add(lblNewLabel);

                textField = new JTextField();
                textField.setBounds(76, 8, 244, 20);
                panel.add(textField);
                textField.setColumns(10);

                JLabel lblLastname = new JLabel("Lastname :");
                lblLastname.setBounds(10, 42, 78, 14);
                panel.add(lblLastname);

                textField_1 = new JTextField();
                textField_1.setBounds(76, 39, 244, 20);
                panel.add(textField_1);
                textField_1.setColumns(10);

                JButton btnValidate = new JButton("Validate");
                btnValidate.addMouseListener(new MouseAdapter() {
                        @SuppressWarnings("deprecation")
                        @Override
                        public void mousePressed(MouseEvent arg0) {
                                JPanel panel_1 = new JPanel();
                                JPanel panel_2 = new JPanel();

                                if(textField.getText().equals("")) {

                                        panel_1.setBackground(new Color(30, 144, 255));
                                        panel_1.setBounds(100, 116, 330, 26);

                                        JLabel lblMessage = new JLabel("0 :");
                                        lblMessage.setForeground(new Color(255, 255, 255));
                                        lblMessage.setFont(new Font("Tahoma", Font.BOLD, 13));

                                        panel_1.add(lblMessage);

                                        frame.getContentPane().add(panel_1);

                                        frame.revalidate();
                                        frame.repaint(10);
                                        frame.revalidate();
                                }
                                else if(textField_1.getText().equals("")) {

                                        panel_2.setBackground(new Color(50, 200, 255));
                                        panel_2.setBounds(10, 134, 330, 26);

                                        JLabel lblMessage = new JLabel("1 :");
                                        lblMessage.setBounds(50, 50, 50, 50);
                                        lblMessage.setAlignmentX(50);
                                        lblMessage.setForeground(new Color(255, 255, 255));
                                        lblMessage.setFont(new Font("Tahoma", Font.BOLD, 13));

                                        panel_2.add(lblMessage);

                                        frame.getContentPane().add(panel_2);

                                        frame.remove(panel_1);

                                        frame.revalidate();
                                        frame.repaint(10);
                                        frame.revalidate();
                                }
                        }
                });

                btnValidate.setBounds(231, 71, 89, 23);
                panel.add(btnValidate);
        }
}

Answer 1

The easiest way is to simply adjust the visibility (JComponent#setVisible( false ) ).

If you really want to remove the component completely, you have to remove and revalidate, as documented in the Container#remove method

This method changes layout-related information, and therefore, invalidates the component hierarchy. If the container has already been displayed, the hierarchy must be validated thereafter in order to reflect the changes.

which results in code like

panel.remove( componentToRemove );
panel.revalidate();
panel.repaint();

As a side note: please replace the null layout and those setBounds call by a proper LayoutManager. You might want to take a look excellent ‘Nested layout example’ available on SO to see what is possible with layout managers. The Swing tag info on SO contains some extra useful links when starting to work with layout managers